Does the extrusion of plasma membrane-enclosed vesicles from the apical surface of isolated choroid plexus (CP) also take place in the intact rat? Two approaches were used. First, the morphological detection of the apically derived vesicles (aposomes) was made in cerebrospinal fluid (CSF) and deep cervical lymph nodes. These lymph nodes were examined because they are part of a major pathway of CSF drainage and it was thought that the nodes would filter out the aposomes. The aposomes were identified immunohistochemically with commercially obtained antiserum to human transthyretin which, in the brain, is synthesized exclusively by the CP and by a donated monoclonal antibody (C3.5) raised against a crude preparation of aposomes and shown to be specific for CP and ependyma. The presence of aposomes in the CSF and deep cervical lymph nodes of the intact rat lend credence to the hypothesis that aposomes are elaborated and released in vivo and suggest that they might be involved in immune responses. Second, protein synthesis in isolated aposomes was described by 2-dimensional SDS-PAGE. Labeled amino acids were used to mark the proteins that were being synthesized and immunoblotting procedures were used to identify important proteins. It was thus determined that, unlike entire choroid plexus tissue, isolated aposomes do not synthesize large amounts of transthyretin but do synthesize two other unidentified proteins. This partitioning of protein synthesis implies a directed cellular sorting by the aposomes and that the selected protein synthesis may have an important bearing on the, as yet, unknown function of aposomes.